Double-sided fresnel zone plates as high performance optics in X-ray microscopy

X-ray microscopy describes a range of analytical techniques, specialized for the characterization of organic and inorganic samples using high energy photons. It takes advantage of the high penetration depth, high resolution and chemical sensitivity of X-rays and allows for the study of extended samples in their native environment without extensive sample preparation. Many of these experimental methods employ diffractive X-ray optics, like Fresnel zone plate lenses to obtain high spatial resolution or the better utilization of the incoming flux. Since improving the efficiency of zone plates can increase the throughput, quality and resolution of measurements, there is a constant demand for high efficiency and high resolution X-ray optics.
Stacking is an established concept for extending the capabilities of zone plate optics. By stacking two zone plates in each other's optical near field, they act as a single zone plate with combined optical transmission profile, that would be infeasible to make as a single optical element. Yet the existing implementations of stacking suffer from issues regarding complexity and stability.
This work presents the development of an alternative solution to conventional zone plate stacking, that circumvents most of its drawbacks. By patterning two zone plates on the front and back sides of a membrane, double-sided zone plates can deliver the advantages of stacked zone plates as inherently monolithic, single-chip optical elements.
Double-sided blazed zone plates with two complementary binary zone plates on the two sides of the membrane were produced to provide an effective four level transmission profile. This allowed to bypass the fundamental limitations of binary zone plates by providing up to 54.7% diffraction efficiency at 6.2 keV while having 200 nm smallest half-pitch and a reasonable working distance.
For high resolution zone plates, structure height is the main limiting factor. Therefore by patterning two identical zone plates on the two sides of the membrane, one can double the effective structure height. This provided us with a significant gain in focusing efficiency at high photon energies, as we have successfully measured 9.9% focusing efficiency at 9 keV with 30 nm smallest half-pitch, while preserving diffraction limited optical performance.
Stacking two complementary zone plates for multiplying their spatial frequencies opens the possibility for ultra-high resolution zone plate optics. We have successfully produced and tested interlaced zone plate optics down to 7 nm smallest half-pitch while still maintaining practical aperture sizes.
This thesis is a comprehensive summary of the work performed for the fabrication and characterization of the high performance zone plates representing each concept and provides possible examples for their future use.